What is the best open-source, natural language processor for doing concept searches?

What is the best open-source, natural language processor for doing concept searches?

I’m going to suggest that the semantic web technologies would be the most developed avenue, certainly in terms of open source. The Wikipedia Category:Semantic Web is probably the best starting point. In fact, the list of Subcategories presents an interesting summary, including:

Folksonomy?
Knowledge bases?
Microformats?
Resource Description Framework?
Triplestores?

References:

Semantic Web Services (2011)